Air Surface Exchange

Background.

ARL conducts basic research on methods for predicting and measuring exchange of meteorological quantities (heat, moisture, and momentum) as well as of trace gases and particles (e.g. ozone, carbon dioxide, sulfur and nitrogen oxides, base cations, nutrients, radioactives, and toxic chemicals) between the atmosphere and various surfaces. The study of trace gas and particle deposition is addressed elsewhere (see "AIRMoN-Dry"). In general, however, the trace gas and particle question is not limited to deposition, but also involves emission and resuspension.

Presently, ARL focuses its attention on the development of systems for measuring fluxes at specific locations, and the extension of local measurements and understanding to describe areal average exchange in numerical models. The work is in support of model developments that increasingly emphasize the sensitivity of predictions to surface properties. It is anticipated that the next generation of forecast models will incorporate a level of detail of air-surface exchange parameterization that will present a challenge to process understanding and measurement technology. The core issue is that contemporary exchange formulations are based on the results of experiments conducted over flat land, usually treeless and unpopulated. Extension of these formulations to the "real world" requires a leap of faith that cannot yet be tested with accuracy.

Why NOAA, why ARL, and why now?

Understanding of the interaction between the air and the earth surface is crucial to all attempts to predict the behavior (or composition) of air near the ground. Improving NOAA's prediction capabilities requires this understanding. A long-term dedicated effort is necessary, in full collaboration with university partners. NOAA has been leading in this work for decades, and ARL is recognized world-wide as the source of NOAA expertise. The next generation of models is being developed now, and the relevant understanding is desired immediately. ARL field research is moving at a fast pace, so that modelers in the outside community do not get too far ahead of current understanding. ARL's internal model developments are arranged to be in close association with the field work.

Tower Studies.

Three ARL groups (Oak Ridge, Research Triangle Park, and Silver Spring) are currently working with portable eddy flux systems, based upon original ARL developments. It is standard operating procedure to measure routinely all of momentum, sensible and latent heat, and carbon dioxide fluxes in all applications of these new systems.

At Oak Ridge, and as part of a continuing effort to improve and streamline flux measurement systems, a tower-based eddy correlation system powered by a 12 volt battery with solar panel (and or wind generator) recharging has been deployed for nearly two years in the Little Washita watershed in Oklahoma. This location is one favored for extensive hydological studies. It is included in the DOE ARM/CART area of intensive radiation studies. The system is specifically designed to provide uninterrupted monitoring of momentum, heat, water vapor, and carbon dioxide fluxes. Data are transferred to the laboratory at Oak Ridge daily, via a cellular telephone system.

The center piece of studies at Oak Ridge is the operation at the Walker Branch watershed field site, where a tall tower extends through the forest canopy and permits monitoring of the same fluxes to a forest canopy. This activity was started in 1994, and will continue into the foreseeable future. A unique record of the seasonal variation of fluxes has been obtained. This already shows the great sensitivity of forest CO2 uptake rates to climatic variables. Linkages with other quantities that control forest growth rates are now being explored.

Walker Branch watershed flux studies have recently been extended in an exploration of the flux contributions of the forest floor and the trees themselves. Two eddy-correlation measurement systems continuously measured energy and CO₂ exchange both at the forest floor and above the canopy, to evaluate the separate (ground and in-canopy) sources of latent heat and CO₂. Experience gained in this effort will be important for anticipated surface-layer model testing and evaluation studies (under NOAA/GEWEX/GCIP).

At Research Triangle Park, and in cooperation with Oak Ridge, a separate portable flux-measuring system was developed, this time designed for direct measurement of trace gas fluxes but relying on measurement of the standard micrometeorological quantities for quality assurance. The system provides for direct eddy correlation measurements of sulfur dioxide, ozone, and carbon dioxide fluxes, and of nitric acid by filter pack gradient analysis, as well as the important components of the surface energy budget. The system has been tested in field programs at Beaufort, NC, and at Bondville, IL.

At Silver Spring, the eddy correlation focus is on nitrogen oxides, in support of studies related to coastal eutrophication. The tower system follows the design principles developed at Oak Ridge, and adopts the same data acquisition and reduction software. The aim of this work is to develop an understanding of the exchange processes in a coastal environment, where the role of ozone is considerable and sea salt adds a further complexity.

During the last year, the three ARL capabilities at Research Triangle Park, Oak Ridge, and Silver Spring have joined forces to attack the vexing question of NO emission from coastal agricultural land. NO from farmland is known to contribute to the coastal ozone problem and also to nitrogen loadings of potentially sensitive aquatic ecosystems. Current assessments are limited by uncertainties about the contribution of agricultural nitrogen emissions relative to NO production by industry and automobiles. The studies in which the ARL teams collaborated were designed to measure NO emission rates, and to test whether chamber methodologies yield biased answers. Preliminary results appear encouraging. There appears to be agreement between chamber and eddy correlation methodologies.

The Mobile Flux Platform, and GPS.

A major ARL development has been that of the Mobile Flux Platform (MFP), a system that corrects velocity records for the effects of using a sampling platform that is not rigidly fixed to the earth surface. During 1994, the use of new Global Positioning System (GPS) technology was evaluated, and the newest available GPS systems were adopted. The MFP GPS systems were upgraded, and a post-processing software from the University of Calgary, was implemented to allow GPS Doppler velocities to be corrected to 2 cm/s (while flying on an aircraft at 50 m/s). Before this modification, platform velocity accuracy was limited to about least 20 cm/s. No other airborne systems are known that have a position accuracy of 3 m, angular accuracy of 1 milliradian, and velocity accuracy of 2 cm/s. ATDD operates a privately-owned Long-EZ aircraft as a test platform for flux developments. The systems developed for aircraft eddy flux use have now been fitted to one of NOAA's two Twin Otter aircraft. The capability that now exists is unique in the US, and is presently matched by only one other in the world.

Large-Area Exchange

The Oak Ridge group has frequently deployed both tower and aircraft eddy correlation systems during studies of areal fluxes over a heterogenous surfaces, in real-world studies of how well flat-earth formulations apply in real situations. An intensive study at Boardman, Oregon (as part of the DOE ARM program) focussed on a farm where mechanical irrigation systems are used to provide water to 800 m diameter circles. Analysis of tower eddy correlation fluxes of heat and moisture displayed differences in the fluxes among alfalfa, corn, and wheat crops; during daytime, transpiration rates differed by 20% to 50%.

Measurements of momentum, heat, and moisture fluxes from the ATDD Long-EZ research airplane were analyzed to quantify spatial variabilities in the fluxes. Fluxes from an 800 m crop circle were compared to average fluxes from transects across the whole farm. On average, a 50 to 100 W/m² difference from field to field has been found during daytime hours. A similar assessment has been planned for the ARM Southern Great Plains (Oklahoma) CART site using modeled sensible and latent heat fluxes. The model will be tested with measurements recently obtained from the SGP site.

Carbon Dioxide.

Continuous eddy correlation measurement of CO₂ flux over the Walker Branch (Oak Ridge) forest have continued since 1993. Annual CO₂ fluxes have been found to exceed the values found over the older and more northern Harvard forest by a factor of about three.

The eddy flux measurement of CO₂ exchange is now a mature technology. Monitoring stations are now operational at the Little Washita watershed in Oklahoma, and at the AIRMoN and ISIS/SURFRAD site at Bondville, Illinois. (See discussion above -- "Tower Studies".)

In recognition of the recent advances in measurement technology, ATDD organized (collaboratively with the University of Tuscia, Italy) a workshop on Strategies for Monitoring CO₂ Fluxes over Terrestrial Ecosystems. The workshop was conducted in December, 1994, in Italy. The goal was to take a first step towards establishing a global network that can directly address the issue of the missing sink in the global carbon balance. Since then, there has been an exploding interest in this topic, and association with the Global Atmosphere Watch of the World Meteorological Organization is now being explored.

CASES

Air-surface exchange has been studied extensively in classical investigations that focus on revealing the processes involved. These studies take place in regions where the factors that control these processes are most clearly observable -- in flat land with homogeneous surface cover. There is a paucity of air-surface exchange data over other kinds of terrain. Contemporary models make universal use of then elationshils developed in these flat-earth studies, even though they may not apply in more general situations. It is necessary to assess the errors that arise, especially since these may be biased towards underestimating evaporation which in turn will afect precipitation forecasts.

NOVA.

From mid-July to mid-August 1996, scientists from NOAA's Air Resources Laboratory participated in the Project NOVA (Natural emissions of Oxidant precursors: Validation of techniques and Assessment) field experiment in rural northeastern North Carolina. The primary purpose of the project was to compare different methodologies for estimating the release of nitric oxide (NO) from agricultural soils. Secondary goals of the project included the quantification of emission estimates of NO from agricultural crop land, and characterization of tropospheric photochemistry at a rural site in the southeastern U.S.

Plans for the Future

The importance of accurate air-surface flux formulation in numerical models is now widely acknowledged. In coming years, the focus of a sizeable fraction of the flux-measuring and PBL-exploring community will be on a watershed located immediately to the east of Wichita, Kansas. The Cooperative Air Surface Exchange Site) CASES is being promoted as a mechanism to concentrate attention on an area that represents a step forward from the terrain simplicity that has attracted workers previously. This is a component of the GEWEX program, in which NOAA plays a leading role. In the immediate future, ARL will be deploying the Twin Otter aircraft and tower instrumentation, in collaboration with scientists from Argonne National Laboratory and NCAR, in a scoping study presently planned for Spring, 1997.